Added example with user data input to the panel mixin doc (#5848)

* Added example with user data input to the panel mixin doc

* Typo

* Added changes proposed by Lukas

docs/docs/extend/plugins/panel.md

@@ -53,7 +53,7 @@ Or to add a template file that will be rendered as javascript code, from the plu
 
 Note : see convention for template directory above.
 
-## Example Implementation
+## Example Implementations
 
 Refer to the `CustomPanelSample` example class in the `./plugin/samples/integration/` directory, for a fully worked example of how custom UI panels can be implemented.
 
@@ -74,9 +74,9 @@ from django.http import HttpResponse
 
 from order.views import PurchaseOrderDetail
 from plugin import InvenTreePlugin
-from plugin.mixins import PanelMixin, SettingsMixin, UrlsMixin
+from plugin.mixins import PanelMixin, UrlsMixin
 
-class MouserCartPanel(PanelMixin, SettingsMixin, InvenTreePlugin, UrlsMixin):
+class MouserCartPanel(PanelMixin, InvenTreePlugin, UrlsMixin):
 
     value=1
 
@@ -147,9 +147,10 @@ async function JGetCart(){
 {% endraw %}
 ```
 
-We start with a bit of javascript. The function JGetCart just calls the url that has been defined in the python code above.
+We start with a bit of javascript. The function JGetCart just calls the url
-The url consists of a full path `plugin:plugin-name:url-name`. The plugin-name is the SLUG that was defined in the plugin code.
+that has been defined in the python code above.  The url consists of a full
-order.pk is the parameter that is passed to python.
+path `plugin:plugin-name:url-name`. The plugin-name is the SLUG that was
+defined in the plugin code. order.pk is the parameter that is passed to python.
 
 The button is defined  with `class="btn btn-info` This is an InvenTree predefined button. There a are lots of others available.
 Here are some examples of available colors:
@@ -160,3 +161,98 @@ Please have a look at the css files for more options. The last line renders the
 
 !!! tip "Give it a try"
     Each time you press the button, the value will be increased.
+
+### Handling user input
+
+A common user case is user input that needs to be passed from the panel into
+the plugin for further processing. Lets have a look at another example. We
+will define two user input fields. One is an integer the other one a string.
+A button will be defined to submit the data. Something like that:
+
+{% with id="panel_with_userinput", url="plugin/panel_with_userinput.png", description="Panel with user input" %} {% include "img.html" %} {% endwith %}
+
+Here is the plugin code:
+
+```python
+from django.urls import path
+from django.http import HttpResponse
+
+from plugin import InvenTreePlugin
+from plugin.mixins import PanelMixin, UrlsMixin
+
+class ExamplePanel(PanelMixin, InvenTreePlugin, UrlsMixin):
+
+    NAME = "ExamplePanel"
+    SLUG = "examplepanel"
+    TITLE = "Example for data input"
+    AUTHOR = "Michael"
+    DESCRIPTION = "This plugin passes user input from the panel to the plugin"
+
+# Create the panel that will display on every view
+    def get_custom_panels(self, view, request):
+        panels = []
+        panels.append({
+            'title': 'Example Info',
+            'icon': 'fa-industry',
+            'content_template': 'example_panel/example.html',
+        })
+        return panels
+
+    def setup_urls(self):
+        return [
+                path("example/<int:layer>/<path:size>/",
+                    self.do_something, name = 'transfer'),
+        ]
+
+# Define the function that will be called.
+    def do_something(self, request, layer, size):
+
+        print('Example panel received:', layer, size)
+        return HttpResponse(f'OK')
+```
+
+The start is easy because it is the same as in the example above.
+Lets concentrate on the setup_urls. This time we use
+path (imported from django.urls) instead of url for definition. Using path makes it easier to
+define the data types. No regular expressions. The URL takes two parameters,
+layer and size, and passes them to the python function do_something for further processing.
+Now the html template:
+
+```html
+{% raw %}
+<script>
+async function example_select(){
+    const layernumber = parseInt(document.getElementById("layer_number").value)
+    const size = document.getElementById("string").value
+    response = inventreeFormDataUpload(url="{% url 'plugin:examplepanel:transfer' '9999' 'Size' %}"
+                                          .replace("9999", layernumber)
+                                          .replace("Size", size)
+                                      );
+}
+</script>
+
+<form> 
+    Number of Layers<br> 
+    <input id="layer_number" type="number" value="2"><br>
+    Size of Board in mm<br>
+    <input id="string" type="text" value="100x160">
+</form>
+
+<input type="submit" value="Save" onclick="example_select()" title='Save Data'>
+{% endraw %}
+```
+
+The HTML defines the form for user input, one number and one string. Each
+form has an ID that is used in the javascript code to get the input of the form.
+The response URL must match the URL defined in the plugin. Here we have a number
+(999999) and a string (Size). These get replaced with the content of the fields
+upon execution using replace. Watch out for the ticks around the 999999 and Size. They prevent
+them from being interpreted by the django template engine and replaced by
+something else.
+
+The function inventreeFormDataUpload is a helper function defined by InvenTree
+that does the POST request, handles errors and the csrftoken.
+
+!!! tip "Give it a try"
+    change the values in the fields and push Save. You will see the values
+    in the InvenTree log.